Precoding a transmission from an antenna array involves applying a set of complex weights to the signals that are to be transmitted from the array's antenna elements, so as to independently control the signals' phase and/or amplitude. This set of complex weights is referred to as a “precoder”. The transmitting node conventionally chooses the precoder to match the current channel conditions on the link to the receiving node, with the aim of maximizing the link capacity or quality. If multiple data streams are simultaneously transmitted from the array's antenna elements using spatial multiplexing, the transmitting node also typically chooses the precoder with the aim of orthogonalizing the channel and reducing inter-stream interference at the receiving node.
In closed-loop operation, the transmitting node selects the precoder based on channel state information (CSI) fed back from the receiving node that characterizes the current channel conditions. The transmitting node in this regard transmits a reference signal from each antenna element to the receiving node, and the receiving node sends back CSI based on measurement of those reference signals. Transmission of the reference signals and feedback of the CSI contribute significant overhead to precoding schemes. For example, these reference signals and CSI feedback consume a significant amount of transmission resources (e.g., time-frequency resource elements in Long Term Evolution, LTE, embodiments).
Known approaches reduce overhead attributable to reference signal transmission by dedicating a reference signal for CSI measurement. LTE Release 10, for example, introduces a CSI Reference Signal (CSI-RS) specifically designed for CSI measurement. Unlike the cell-specific common reference signal (CRS) in previous LTE release, the CSI-RS is not used for demodulation of user data and is not precoded. Because the density requirements for data demodulation are not as stringent for CSI measurement, the CSI-RS can be relatively sparse in time and frequency, thereby reducing the number of transmission resources required for transmitting the CSI-RS.
Known approaches reduce overhead attributable to CSI feedback by limiting the usable precoders to a fixed set of precoders, i.e., a codebook. Each precoder in the codebook is assigned a unique index that is known to both the transmitting node and the receiving node. The receiving node determines the “best” precoder from the codebook, and feeds back the index of that precoder (often referred to as a “precoding matrix indicator”, PMI) to the transmitting node as a recommendation (which the transmitting node may or may not follow). Feeding back only an index, in conjunction with other CSI such as the recommended number of data streams (i.e., transmission rank) for spatial multiplexing, reduces the number of transmission resources required for transporting that CSI. This approach therefore reduces CSI feedback overhead considerably as compared to explicitly feeding back complex valued elements of a measured effective channel.
Despite this, overhead from closed-loop precoding remains problematic as antenna array technology advances towards more and more antenna elements. This antenna element escalation stems not only from increases to the number of elements in the traditional one-dimensional antenna array, but also from adoption of two-dimensional antenna arrays that enable beamforming in both the vertical and horizontal spatial dimension. Furthermore, although a codebook reduces CSI overhead, the effective channel quantization inherent in the codebook has heretofore limited the codebook's ability to flexibly adapt to different propagation environments.